This invention relates to power systems and more particularly to DC-to-AC switching power converters having reduced power consumption and reduced electromagnetic interference.
Due to the widespread use of switchmode power supplies, utility AC power systems have to deliver power to an increasing number of non-linear loads resulting in low power factor of the utility systems. These non-linear loads create significant electromagnetic interference in the harmonic currents drawn from the utility power buses. In addition to the unnecessary losses in power transmission due to the presence of these harmonic currents, the utility systems are polluted since conductive and radiated electromagnetic interference can propagate and degrade the performance of other sensitive electronic equipments or appliances sharing the same power bus.
Conventional approaches use passive line filters to attenuate these interferences. These approaches are no longer effective because bulky components are needed to absorb the harmonic currents and the fundamental component of the currents still have higher RMS value than necessary.
The preferred remedy for attenuating interference is active power factor correction in which switchmode converter topologies are utilized. A boost converter is the best topology for this application because it can be operated to draw continuous current with much less harmonics, resulting in ease of line-filtering and is more effective in the reduction of electromagnetic interference (EMI).
In the conventional DC boost converter topology, power transfer to the load is accomplished without electrical isolation from line to output since its output rectifier is a passive switch which cannot prevent a transformer inserted between the rectifier output and load circuit from saturating. The lack of electrical isolation makes it impossible to achieve a step-down output voltage in the single stage of power conversion.
Another type of boost converter configurations is the push-pull configuration. In the push-pull converter configurations, such as the converter described in U.S. Pat. No. 4,885,675, an isolation transformer is required to have two windings at the primary side and full-wave rectification at the secondary side so as to operate the transformer symmetrically without core saturation. However, the voltage stresses on the push-pull switching devices are twice the reflected output voltage at either side of the primary windings. Consequently, the push-pull boost converter will sacrifice more costly switching devices in order to achieve the same conduction losses yielded from the conventional boost converter. For example, a push-pull boost converter with nearly unity power factor used in a 200 volt AC system will require active switching devices having as high as 1000 volt breakdown voltage. The on-resistance of such switching devices is significantly high, causing more conduction losses. Further, a push-pull topology for DC-to-DC conversion requires at least three windings, two on the primary and one on the secondary.